Method and system for providing variable ramp-up control for an electric heater

ABSTRACT

In one form, the present disclosure is directed toward a method for controlling temperature of a heater including a resistive heating element. The method includes applying power to the resistive heating element of the heater at a variable ramp rate to increase temperature of the heater to a desired temperature setpoint. The variable ramp rate is set to a desired ramp rate. The method further includes monitoring an electric current flowing through the resistive heating element of the heater, and reducing the variable ramp rate from the desired ramp rate to a permitted ramp rate in response to the electric current being greater than a lower limit of an electric current limit band. An upper limit of the electric current limit band is provided as a system current limit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalApplication 63/064,523 filed on Aug. 12, 2020. The disclosure of theabove application is incorporated herein by reference.

FIELD

The present disclosure relates to controlling the temperature of aheater.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

A thermal system generally includes a heater having resistive heatingelements and a control system for controlling power to the heater togenerate heat at a temperature setpoint. In an example application, asemiconductor process system includes a thermal system having a pedestalheater that includes a heating plate with a ceramic substrate and one ormore resistive heating elements that define one or more heating zones.The pedestal heater can be heated to different temperature setpoints toperform various processes such as heating a semiconductor wafer, acleaning cycle, and among other operations.

To reach the temperature setpoint, the control system typically ramps upthe temperature at a standard ramp rate (e.g., 5° C./min, 10° C./min,among others). The time spent changing the temperature setpointtypically idles a semiconductor chamber having the heater, which is lostor non-productive manufacturing time. These and other issues related toadjusting temperature of a heater are addressed by the presentdisclosure.

SUMMARY

This section provides a general summary of the disclosure and is not acomprehensive disclosure of its full scope or all of its features.

In one form, the present disclosure is directed to a method ofcontrolling temperature of a heater including a resistive heatingelement, and the method includes applying power to the resistive heatingelement of the heater at a variable ramp rate to increase temperature ofthe heater to a desired temperature setpoint, where the variable ramprate is set to a desired ramp rate. The method further includesmonitoring an electric current flowing through the resistive heatingelement of the heater and reducing the variable ramp rate from thedesired ramp rate to a permitted ramp rate in response to the electriccurrent being greater than a lower limit of an electric current limitband, where an upper limit of the electric current limit band isprovided as a system current limit.

In one variation, reducing the variable ramp rate further includesdetermining a reduction amount of the desired ramp rate based on avariable reduction factor, where the variable reduction factor increasesas the electric current of the resistive heating element approaches thesystem current limit and decreasing the variable ramp rate by thereduction amount to the permitted ramp rate.

In another variation, the variable reduction factor provides a scaledreduction of the variable ramp rate based on proximity of the electriccurrent to the system current limit.

In yet another variation, wherein the reduction amount is determinedbased on the following: RedAmt=(DesiredRate*% Reduction*RedFactor),where Reduction=1.0−((ZoneCurLim−MeasuredCurrent)/CurrentBand) in which:“RedAmt” is the reduction amount, “DesiredRate is the desired ramp rate,“RedFactor” is amount the variable ramp rate is reduced when theelectric current is equal to the system current limit, “ZoneCurLim” ismaximum electric current limit for the resistive heating element,“MeasuredCurrent” is the electric current that is measured, and“CurrentBand” is the electric current limit band.

In one variation, the heater includes a plurality of resistive heatingelements that define a plurality of zones, where each of the pluralityof zones has a defined variable ramp rate.

In another variation, the electric current at each of the plurality ofzones is monitored, and the variable ramp rate is reduced from thedesired ramp rate to the permitted ramp rate in response to at least onezone of the plurality of zones having an electric current that isgreater than the lower limit of the electric current limit band.

In yet another variation, the method further includes determining areduction amount for the at least one zone having the electric currentgreater than the lower limit of the electric current limit band based ona variable reduction factor, where the variable reduction factorincreases as the electric current approaches the system current limit,and reducing the variable ramp rate for each of the plurality of zonesbased on the reduction amount to obtain the permitted ramp rate for eachof the plurality of zones.

In one variation, the method further includes monitoring a zonetemperature for each of the plurality of zones, determining whether adifference between a first zone temperature of a first zone from amongthe plurality of zones and a second zone temperature of a second zonefrom among the plurality of zones is greater than a deviation threshold,and adjusting the variable ramp rate for the first zone, the secondzone, or in combination thereof in response to the difference beinggreater than the deviation threshold, where a zone from among the firstzone and the second zone having a higher zone temperature is provided asa hot zone and the other among the first zone and the second zone is acool zone.

In another variation, to adjust the variable ramp rate, the variableramp rate for the hot zone is reduced, the variable ramp rate of thecool zone is increased, or a combination thereof.

In yet another variation, to adjust the variable ramp rate, the methodfurther includes reducing the variable ramp rate of the hot zone to zeroto hold the zone temperature of the hot zone until the difference is nolonger greater than the deviation threshold, and increasing the variableramp rate of the hot zone in response to the difference being less thanthe deviation threshold.

In one variation, the method further includes setting the variable ramprate to a glide control rate, where the glide control rate is less thanthat of the desired ramp rate, and increasing the variable ramp rate tothe desired ramp rate in response to a glide condition being satisfied,where the glide condition include a predetermined time passing, thetemperature of the heater equaling a glide temperature setpoint that isless than the desired temperature setpoint, or a combination thereof.

In another variation, the method further includes determining whetherthe temperature of the heater is at a temperature approach threshold,where the temperature approach threshold is less than the desiredtemperature setpoint, and decreasing the variable ramp rate to anapproach ramp rate in response to the temperature of the heater being atthe temperature approach threshold, where the approach ramp rate is lessthan the desired ramp rate.

In one form, the present disclosure is directed to a control system forcontrolling power to a heater including a resistive heating element, andthe control system includes a processor and a nontransitorycomputer-readable medium including instructions that are executable bythe processor, where the instructions include determining amount ofpower to be provided to the resistive heating element of the heaterbased on a variable ramp rate to increase temperature of the heater to adesired temperature setpoint, where the variable ramp rate is set to adesired ramp rate. The instructions further include monitoring anelectric current flowing through the resistive heating element of theheater and reducing the variable ramp rate from the desired ramp rate toa permitted ramp rate in response to the electric current being greaterthan a lower limit of an electric current limit band, where an upperlimit of the electric current limit band is provided as a system currentlimit.

In one variation, the instructions further include determining areduction amount of the desired ramp rate based on a variable reductionfactor, where the variable reduction factor increases as the electriccurrent of the resistive heating element approaches the system currentlimit, and decreasing the variable ramp rate by the reduction amount toobtain the permitted ramp rate.

In another variation, the variable reduction factor provides a scaledreduction of the variable ramp rate based on proximity of the electriccurrent to the system current limit.

In yet another variation, the heater includes a plurality of resistiveheating elements that define a plurality of zones, where each of theplurality of zones has a defined variable ramp rate.

In one variation, the electric current at each of the plurality of zonesis monitored, and the variable ramp rate is reduced from the desiredramp rate to the permitted ramp rate in response to at least one zone ofthe plurality of zones having an electric current that is greater thanthe lower limit of the electric current limit band.

In another variation, the instructions further includes determining areduction amount for the at least one zone having the electric currentgreater than the lower limit of the electric current limit band based ona variable reduction factor, where the variable reduction factorincreases as the electric current approaches the system current limit,and reducing the variable ramp rate for each of the plurality of zonesbased on the reduction amount to obtain the permitted ramp rate for eachof the plurality of zones.

In yet another variation, the instructions further include monitoring azone temperature for each of the plurality of zones, determining whethera difference between a first zone temperature of a first zone from amongthe plurality of zones and a second zone temperature of a second zonefrom among the plurality of zones is greater than a deviation threshold,and adjusting the variable ramp rate for the first zone, the secondzone, or in combination thereof in response to the difference beinggreater than the deviation threshold, where a zone from among the firstzone and the second zone having a higher zone temperature is provided asa hot zone and the other among the first zone and the second zone is acool zone.

In one variation, to adjust the variable ramp rate, the instructionsfurther include reducing the variable ramp rate for the hot zone,increasing the variable ramp rate of the cool zone, or a combinationthereof.

In another variation, the instructions further include reducing thevariable ramp rate of the hot zone to zero to hold the zone temperatureof the hot zone until the difference is no longer greater than thedeviation threshold, and increasing the variable ramp rate in responseto the difference being less than the deviation threshold.

In yet another variation, the instructions further include setting thevariable ramp rate to a glide control rate, where the glide control rateis less than that of the desired ramp rate, and increasing the variableramp rate to the desired ramp rate in response to a glide conditionbeing satisfied, where the glide condition include a predetermined timepassing, the temperature of the heater equaling a glide temperaturesetpoint that is less than the desired temperature setpoint, or acombination thereof.

In one variation, the instructions further include determining whetherthe temperature of the heater is at a temperature approach threshold,where the temperature approach threshold is less than the desiredtemperature setpoint, and decreasing the variable ramp rate to anapproach ramp rate in response to the temperature of the heater being atthe temperature approach threshold, where the approach ramp rate is lessthan the desired ramp rate.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now bedescribed various forms thereof, given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 illustrates a thermal system having a heater and control systemwith a variable ramp rate temperature control in accordance with thepresent disclosure;

FIG. 2 is a flowchart of an exemplary variable ramp rate temperaturecontrol in accordance with the present disclosure;

FIG. 3 is a flowchart of an example variable ramp-up control of FIG. 2;

FIG. 4 is a flowchart of an example variable ramp-down control of FIG.2;

FIG. 5A is a graph a constant ramp-up control in accordance with thepresent disclosure;

FIG. 5B is a graph of a variable ramp-up control in accordance with thepresent disclosure;

FIG. 6 is a graph of a variable ramp-up control in accordance with thepresent disclosure; and

FIGS. 7A and 7B are graphs of variable ramp-down control in accordancewith the present disclosure;

FIG. 8 is a graph of a variable ramp-down control to inhibit zone driftcondition in accordance with the present disclosure; and

FIG. 9 is a graph of the variable ramp-down control to mitigate runawayconditions in accordance with the present disclosure.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

Referring to FIG. 1, a thermal system 100 includes a pedestal heater 102and a control system 104 having a controller 106 and a power convertersystem 108. In one form, the heater 102 includes a heating plate 110 anda support shaft 112 disposed at a bottom surface of the heating plate110. The heating plate 110 includes a substrate 111 and a plurality ofresistive heating elements (not shown) embedded in or disposed along asurface of the substrate 111 (i.e., “plurality” means two or more). Inone form, the substrate 111 may be made of ceramic or aluminum. Theresistive heating elements are independently controlled by the controlsystem 104 and define a plurality of heating zones 114 as illustrated bythe dashed-dotted lines in FIG. 1. It is readily understood that theheating zones 114 could take a different configuration while remainingwithin the scope of the present disclosure. In addition, the pedestalheater 102 may include one or more zones and should not be limited to amultizone heater.

In one form, the heater 102 is a “two-wire” heater in which theresistive heating elements function as heaters and as temperaturesensors with only two leads wires operatively connected to the heatingelement rather than four. Such two-wire capability is disclosed in, forexample, U.S. Pat. No. 7,196,295, which is commonly assigned with thepresent application and incorporated herein by reference in itsentirety. Typically, in a two-wire system, the resistive heatingelements are defined by a material that exhibits a varying resistancewith varying temperature such that an average temperature of theresistive heating element is determined based on a change in resistanceof the resistive heating element. In one form, the resistance of theresistive heating element is calculated by first measuring the voltageacross and the electric current through the heating elements and then,using Ohm's law, the resistance is determined. Using aresistance-temperature conversion data (e.g., a table, an algorithm,among others), a temperature of the resistive heating element and thus,the zone 114 is determined (i.e., a zone temperature). The resistiveheating element may be defined by a relatively high temperaturecoefficient of resistance (TCR) material, a negative TCR material, or inother words, a material having a non-linear TCR.

The control system 104 controls the operation of the heater 102, andmore particularly, is configured to independently control power to eachof the zones 114. In one form, the control system 104 is electricallycoupled to the zones 114 via terminals 115, such that each zone 114 iscoupled to two terminals providing power and sensing temperature.

In one form, the control system 104 is communicably coupled (e.g.,wireless and/or wired communication) to a computing device 117 havingone or more user interfaces such as a display, a keyboard, a mouse, aspeaker, a touch screen, among others. Using the computing device 117, auser may provide inputs or commands such as temperature setpoints, powersetpoints, and/or commands to execute a test or a process stored by thecontrol system 104.

The control system 104 is electrically coupled to a power source 118that supplies an input voltage (e.g., 240V, 208V) to the power convertersystem 108 by way of an interlock 120. The interlock 120 controls powerflowing between the power source 118 and the power converter system 108and is operable by the controller 106 as a safety mechanism to shut-offpower from the power source 118. While illustrated in FIG. 1, thecontrol system 104 may not include the interlock 120.

The power converter system 108 is operable to adjust the input voltageand apply an output voltage (V_(OUT)) to the heater 102. In one form,the power converter system 108 includes a plurality of power converters(not shown) that are operable to apply an adjustable power to theresistive heating elements of a zone 114. One example of such a powerconverter system is described in U.S. Pat. No. 10,690,705 titled “POWERCONVERTER FOR A THERMAL SYSTEM”, which is commonly owned with thepresent application and the contents of which are incorporated herein byreference in its entirety. In this example, each power converterincludes a buck converter that is operable by the controller 106 togenerate a desired output voltage that is less than or equal to theinput voltage for one or more heating elements of a given zone 114.Accordingly, the power converter system 108 is operable to provide acustomizable amount of power (i.e., a desired power) to each zone 114 ofthe heater 102. Other power converter systems configured to provideadjustable power to the heater 102 may also be used and should not belimited to the example provided herein. For example, the power convertersystem may be an isolated power converter system for providing anisolated power output to the heater. One example of such a powerconverter system is described in U.S. Pat. No. 11,038,431 titled“ISOLATED POWER CONVERTER FOR A THERMAL SYSTEM”, which is commonly ownedwith the present application and the contents of which are incorporatedherein by reference in its entirety.

With the use of a two-wire heater, the control system 104 includessensor circuits 124 to measure electrical characteristics of theresistive heating elements (i.e., voltage and/or current), which is thenused to determine performance characteristics of the zones 114, such asresistance, temperature, current, voltage, power, and other suitableinformation. In one form, a given sensor circuit 124 includes an ammeter126 and a voltmeter 128 to measure an electric current flowing throughand a voltage applied to the heating element(s) in a given zone 114,respectively. In another form, the voltage and/or current measurementsmay be taken at zero-crossing, as described in U.S. Pat. No. 7,196,295.

In lieu of or in addition to a “two-wire heater”, the thermal system 100may include discrete sensors for measuring characteristics of the heater102 (e.g., voltage, current, and/or temperature) and provide respectivedata to the controller 106. For example, in one form, at least onevoltmeter and ammeter may be provided to measure electricalcharacteristics (e.g., voltage and current) of the zone 114 and at leastone temperature sensor may be provided to measure a temperature of theheater and/or temperature of each zone 114.

In one form, the controller 106 includes one or more microprocessors andmemory for storing computer readable instructions executed by themicroprocessors. In one form, the controller 106 is configured toperform one or more control processes in which the controller 106determines the desired power to be applied to the zones 114, such as100% of input voltage, 90% of input voltage, etc. Example controlprocesses are described in U.S. Pat. No. 10,690,705 (referenced above),and U.S. Pat. No. 10,908,195 titled “SYSTEM AND METHOD FOR CONTROLLINGPOWER TO A HEATER, which is commonly owned with the present applicationand the contents of which are incorporated herein by reference in itsentirety. In one form, the controller 106 performs a closed-looptemperature control in which the temperature of the heater is controlledto a temperature setpoint. For example, using the resistance of theresistive heating elements and a calibrated resistance-temperaturemodel, the controller 106 determines a temperature of the zones 114 andthen adjusts the power to the zones 114 to bring the temperature of thezones 114 closer to the temperature setpoint.

In one form, the control processes also includes a variable ramp ratetemperature (VRRT) control 130 in which the heater 102 initiallyundergoes a variable temperature ramp rate to reach a temperaturesetpoint. Once at the temperature setpoint, the controller provides asteady-state closed loop control to maintain the temperature of theheater 102 at the temperature setpoint. In certain applications, theheater 102 may be controlled to different temperature setpoints for anindustrial process and at times, temperatures may fluctuate and go froma first temperature to a second temperature that is much lower than thefirst temperature.

In one form, the VRRT control 130 is configured to provide a variableramp-up control to increase the temperature of the heater 102 and avariable ramp-down control to reduce the temperature of the heater 102.While the VRRT control 130 is provided as having both, the VRRT control130 can include one of the variable ramp-up control and the variableramp-down control and is not required to have both.

The variable ramp-up control is configured to provide power to theresistive heating element of the heater 102 at a variable ramp-up rateto increase temperature of the heater 102 to the temperature setpoint.The variable ramp-up rate is defined based on the electric currentprovided to the heater 102 and for multizone heaters, temperature of thezones 114. More particularly, to inhibit damage to components of thethermal system 100 such as a power switches, power converters, wiring,and/or fuses, among others, the electric current applied to the heater102 is controlled below a system current limit, which may be a zonecurrent limit and/or heater current limit. For example, with the zones114, the electric current to each zone 114 is monitored and controlledbelow a zone current limit, as the system current limit, for each zone114. In one form, with respect to a multizone heater, the electriccurrent at one zone may affect the electric current at the other zones.That is, to provide coherent ramping, when a single zone approaches thesystem current limit, the variable ramp-up control adjusts (e.g.,decreases) the variable ramp-up rate for all the zones by the same rateof reduction. For a temperature setpoint, the variable ramp-up controldefines a system current limit (i.e., maximum allowed current for theheater and/or the zones) and a desired ramp rate that is a maximumdesired ramp rate for the variable ramp-up rate.

To provide a coherent temperature profile for a multizone heater, thevariable ramp-up control monitors and controls the temperature of thezones 114 such that the temperature difference between any two zones 114(e.g., a first zone and a second zone) is less than a deviationthreshold (a zone-to-zone drift/deviation). More particularly, rampingis managed by a moving setpoint (i.e., a temperature ramping setpoint(TempRampSP)) that moves at a rate setpoint (RateSP, i.e., a variableramp rate). That is, in one form, the rate setpoint is in ° C. perminute and is the rate at which the TempRampSP changes. The TempRampSPis an absolute temperature that the controller 106 holds a measuredtemperature to as it moves using, for example,proportional-integral-derivative (PID) control. The measured temperaturemay be referred to as a process value (PV). Since the TempRampSP isconstantly moving until it reaches the temperature setpoint, the processvalue should also move. In one form, the integral time constant in thePID is responsive to build the power to match the rate setpoint. In oneform, if the process variable of any one zone deviates from the processvariables of the other zone(s) 114, the variable ramp-up control adjuststhe RateSP of one or more zones to provide a coherent temperaturecontrol of the heater 102. In one form, the variable ramp-up control maydecrease the RateSP of the zone deviating from other zones to providecoherent temperature profile. In another form, the variable ramp-upcontrol may increase the RateSP of the other zone(s) to boostperformance of those zones 114 while monitoring the electric current tothe zones 114.

For the variable ramp-up control, table 1 provides control variablesemployed to control the ramp rate based on current and temperature:

Abbreviation Variable Definition (unit) Desired The rate the temperatureshould ramp at Desired Rate Ramp when it can and is typically fastest (°C./min) Rate achievable rate. Variable Selected ramp rate. RateSP Ramp(° C./min) Rate Zone Maximum allowable current for the ZoneCurLimCurrent zone. (Amps) Limit Heater Maximum allowable current to theHeatCurLim Current heater. (Amps) Limit Current Defines an electriccurrent range close to CurrentBand Limit the system current limit (e.g.,zone (Amps) Band current limit) and once within the range, the ramp rateis reduced. The range includes a lower limit and an upper limit. Theupper limit can be set to the system current limit. The range is definedto be close to the system current limit but large enough to keep theramp rate from passing the system current limit. Reduction Amount theramp rate is reduced by RedFactor Factor when the electric current isequal to system current limit. Reduction The amount the ramp rate isreduced by. RedAmt Amount (° C./min) Tem- Absolute temperature that thezone is TempRampSP perature controlled to as it moves to the (° C.)Ramping temperature setpoint. Setpoint Tem- Setpoint that the zone isbeing ultimately TempSP perature controlled to. (° C.) Setpoint

In one form, to control the ramp-rate based on current, the variableramp-up control sets the variable ramp rate for a zone based on ameasured current for the zone and the total current to the heater. Inparticular, the variable ramp rate is set to be just high enough to stayunder the system current limit (e.g., the zone current limit and/or thezone current limit). The variable ramp rate is initially set to thedesired ramp rate, and if the zone current limit is within the electriccurrent limit band, the variable ramp rate is reduced from the desiredramp rate to a permitted ramp rate based on a calculated reductionamount. In addition to the zone approaching the zone current limit, thevariable ramp rate of the other zones is reduced by the same reductionamount to provide coherent current control. The reduction amount isdependent on how close the measured current is to the system currentlimit such that the smaller the difference between the measured currentand the system current limit the higher the reduction amount.

More particularly, the variable ramp-up control defines a scaledreduction amount for the electric current limit band that is based on apercentage of the reduction factor and a difference between the measuredcurrent and the system current limit such as the zone current limit.That is, an example application, the scaled reduction is based on theproximity of the electric current to the system current limit. Forexample, the reduction amount is determined using equations 1 and 2 inwhich the “% Reduction” is provided as a variable reduction factor thatincreases as the measured current of the resistive heating element for azone approaches the zone current limit.

RedAmt=(DesiredRate*% Reduction*RedFactor)  Equation 1

% Reduction=1.0−((ZoneCurLim−MeasuredCurrent)/CurrentBand)  Equation 2

As provided in equation 2, the variable reduction factor is configuredto provide a scaled reduction such that the reduction parameter is 0% ifthe measured current is below the electric current limit band, between0-100% if the measured current is within the electric current band, 100%if the measured current is equal to the system current limit, andgreater than 100% if the measured current is greater the system currentlimit to provide even more reduction than the reduction factor. In oneform, in the event the measured current is above the zone current limit,the variable ramp rate continues to decrease to a nominal rate such as1° C./min or other suitable value to prevent stall-out.

In one form, to control the ramp-rate based on temperature, the variableramp-up control measures the temperature of each zone and initially setsthe temperature ramping setpoint of a zone to a respective measuredtemperature value to inhibit jumps in temperature. From this point, thetemperature will begin to increase towards the temperature setpoint. Thetemperature of the zones is routinely measured and if the temperature ofa zone begins to deviate from the other zones (i.e., too high or toolow), the variable ramp-up rate is adjusted to provide coherenttemperature. In one form, the variable ramp-up control reduces the ramprate of the zone that is closest to the temperature setpoint (i.e., hotzone) to allow the other zones (i.e., cool zone(s)) to catch-up to thetemperature ramping setpoint of the hot zone. The amount of reduction isselected to provide a responsive reduction, but is not too aggressive soas to reduce the heating operation. For example, the ramp rate may bedecreased by 5-15% for every degree of deviation. In another form, whilemonitoring the electric current to the heater and zones, the variableramp-up control increases the ramp rate of the cool zone(s) to allow thecool zone(s) to catch-up to the temperature ramping setpoint of the hotzone. For example, the ramp rate for the cool zone(s) may be increasedin set incremental amounts (e.g., increase of 1° C./min, 2° C./min, 0.5°C./min). In this boost method, the variable ramp-up control may alsoreduce the ramp rate of the hot zone or hold the temperature of the hotzone to the present temperature ramping setpoint until the other zonesare at or close to the measured temperature of the hot zone.

In one form, at the start of the control, the variable ramp-up controlmay provide a glide control to control how fast the ramp rate changesand an approach control when the temperature setpoint is beingapproached to reduce or inhibit a spike in temperature. Moreparticularly, the ramp rate is set to a glide control rate, which is asignificantly lower ramp-rate than the desired ramp rate (e.g., glidecontrol rate=1.0° C./min). In one form, the ramp rate is maintained atthe glide control rate until a glide condition is satisfied, where theglide condition can include, for example, a predetermined time and/or adesired temperature ramp setpoint (i.e., a glide temperature setpoint)is reached. After which, the variable ramp rate is increased to thedesired ramp rate. In one form, the glide control rate is appliedanytime the ramp rate changes to manage the acceleration of the ramprate.

The approach control is configured to reduce the ramp rate to anapproach ramp rate when the measured temperature is a defineddistance/range (i.e., a temperature approach threshold) from the finaltemperature setpoint. The ramp rate is reduced to allow the heater toreach the temperature setpoint without overshooting the temperaturesetpoint. In one form, the approach control is applied when approachingthe temperature setpoint (e.g., during ramp-up or ramp down) to give theintegral time to wind to a value appropriate to the temperaturesetpoint. For example, if the factor is 1.0, the reduction starts at therate number of degrees away from temperature setpoint. Accordingly, areduction of 10° C./minute, begins to reduce 10° C. away fromtemperature setpoint.

The variable ramp-down control is configured to provide a coherent cooldown of the heater to a temperature setpoint that is less than themeasured temperature. For the semiconductor process, the rate at whichthe heater cools may be a function of the chamber and the rate candecrease as the temperature decreases and/or when walls of the chamberare heated. For a multizone heater, different zones of the heater maycool at different rates when power is removed or significantly reduced.To reduce the temperature difference between the zones, the variableramp-down control is configured to keep the variable ramp rate at orabove the natural fall rate (i.e., reduction rate with no power).

In one from, the variable ramp-down control decreases the temperature ofthe zones at a cooling variable ramp rate such that the temperaturesetpoint continuously decreases at a defined rate. For example, in oneform, the cooling variable ramp rate is first set to a desired coolingramp rate such as 10° C./min and the temperatures of the zones aremonitored to maintain a coherent thermal profile of the heater duringcool down.

To provide the coherent thermal profile, the variable ramp-down controldetermines whether one or more of the following runaway conditions ispresent: a zone-to-zone drift, a ramp setpoint deviation, and/or zonefloating condition. If a runaway condition is detected, the variableramp-down control performs a corrective action.

For the zone-to-zone drift, the variable ramp-down control determineswhether a zone is cooling faster or slower than the other zones.Specifically, in one form, the variable ramp-down control determineswhether a temperature of a subject zone is within a zone deviationthreshold from the other zones. To reduce the deviation and provide acoherent ramp down, if the subject zone is deviating from one or moreother zones, the variable ramp rate for all of the zones is adjusted, asthe corrective action.

For the ramp setpoint deviation, the variable ramp-down controldetermines whether a zone lags too far from the temperature rampingsetpoint while ramping down. Specifically, during ramp-down, thetemperature ramping setpoint is continuously decreasing in accordancewith the variable ramp rate. If the temperature of the subject zone isfalling behind (i.e., not cooling fast enough), the ramp rate isadjusted such that the temperature of the subject zone continues todecrease while allowing the subject zone to catch up to the temperatureramping setpoint. In form, to detect a ramp setpoint deviation, thevariable ramp-down control determines if the temperature of the subjectzone deviates from the temperature ramping setpoint by a value greaterthan or equal to a setpoint deviation threshold (i.e., a deviationthreshold). If so, a ramp setpoint deviating condition is detected.

To mitigate a zone-to-zone drift and/or a ramp setpoint deviation, thevariable ramp-down control reduces the variable ramp rate to a valueless than that of the desired ramp rate (e.g., from 10° C./min to 5°C./min), as the corrective action. In one form, the variable ramp-downcontrol determines the amount of reduction (i.e., a ramp coolingreduction amount (RCoolRedAmt)) based on the amount of deviation betweenthe temperature of the zone to the other zone and/or the temperatureramping setpoint. For example, in one form, the reduction amount isdetermined using equations 3-5 in which: PVH is measured temperature ofthe hot zone; PVL is measured temperature of the cool zone; WeightPara1is a weighted parameter for the delta measured temperatures and isprovided as the amount of reduction per degree of deviation (e.g., 10%PC); and WeightPara2 is a weighted parameter for the difference betweenthe cool zone and the temperature ramp setpoint, and is provided as theamount of reduction per degree of deviation (e.g., 5%/° C.). Oncedetermined, the ramp cooling reduction amount is applied to each zone ofthe zoner.

RCoolRedAmt=Zone Deviation Reduction+Setpoint DeviationReduction  Equation 3

Zone Deviation Reduction=|(PVH−PVL)|*WeightPara1  Equation 4

Setpoint Deviation Reduction=|(PVL−TempRampSP)|*WeightPara2  Equation 5

In one variation, the ramp cooling reduction amount is based on one ofthe zone deviation reduction or the setpoint deviation reduction (i.e.,setpoint deviation amount). For example, if there is only a zone-to-zonedrift, then the setpoint deviation reduction may not be necessary.Alternatively, if both deviation conditions are present, the variableramp-down control may first reduce the deviation of the zone-to-zonedrift based on the zone deviation reduction and until the deviationbetween the zones is within a threshold. After which, the ramp coolingreduction amount is determined using both the zone deviation reductionand setpoint deviation reduction as provided in equation 3. It should bereadily understood that the numerical values provided herein are forexplanation purposes only and can be any suitable value.

In another form, if the temperature of at least one zone begins todeviate from the other zones, the temperature ramping setpoint of thecool zone(s) is set to the measured temperature of the hot zone. Thatis, the variable ramp-down control increases power to the zone with thelower temperature to increase the temperature of the zone to that of thezone having the higher temperature Accordingly, the variable ramp-downcontrol keeps the temperature of the zones together or within adeviation threshold (e.g., ±5° C.) which may flatten the temperatureramping setpoint curve before zones approach the temperature setpoint.

In the zone floating condition, the variable ramp-down controldetermines if a zone is floating or wandering. More particularly, aspower decreases to the zone(s), it can be difficult to accuratelymeasure the process value (e.g., temperature) and in some situations thepower may be so low that the zone may be uncontrollable (e.g., power isat a minimum power level/output that is greater than zero volts, but isinsufficient to control the zone). That is, the temperature of the zonemay begin to deviate from the temperature ramping setpoint and if thereare multiple zones, the temperature of the zone can begin to deviatefrom another zone. To control ramp-down during the zone floatingcondition, the variable ramp-down control is configured to increasepower to the zone undergoing the floating condition to a nominal poweroutput (e.g., 2% power, 5% power) that is greater than the minimum powerlevel (i.e., minimum power output) to obtain control of the zone whilestill decreasing the temperature of the zone. In one form, power isincreased by reducing the variable ramp set point until power is onceagain applied at the nominal power output. The nominal power outputapplied to the zone to inhibit the zone floating condition can bedefined based on testing and may be just above the minimum power level(e.g., nominal power output is above 5V).

In the event more than one of the runaway conditions are detected, thereduction amount is a weighted combination of the reduction amount forthe deviation conditions detected. In one form, the weight assigned foreach deviation condition can be based on what stage the heater is at inthe cool down process. That is, typically, ramp setpoint deviationoccurs earlier of a cool down of the heater than the zone-to-zone drift,which may occur as the heater gets colder. Accordingly, the reductionamount associated with the ramp setpoint deviation is assigned a higherweight than the reduction amount associated with the zone-to-zone driftwhen the heater is first beginning to cool down. After some time and/orafter the temperature of the heater reaches a selected temperaturesetpoint greater than the desired temperature setpoint, the variableramp-down control may assign a higher weight to the reduction amountassociated with the zone-to-zone drift than the ramping setpointdeviation. \At cooler temperatures, power to the heater may no longer beneeded, so a minimal amount of power may be applied to inhibit zonefloating condition, which may take precedent over the zone-to-zone driftand the ramp setpoint deviation. Accordingly, weighted factors can beassigned based on the stage of the heater during the cool down and onthe heater itself (i.e., responsiveness of the heater).

It should be readily understood that the variable ramp-down control canbe configured to monitor one or more runaway conditions, and is notrequired to monitor all. For example, for single zone heater, thezone-to-zone drift is not required.

Referring to FIG. 2, an example VRRT control routine 200 is provided andperformed by the control system to control the temperature of the heaterto one or more temperature setpoints. At 202, the control systemacquires the temperature setpoint for the heater from, for example, adefined state mode that provides temperature setpoints and durations forthe heater. At 204 determines if the temperature setpoint is less thanthe present temperature of the heater. If the temperature setpoint ishigher, the control system performs the variable ramp-up control at 206.On the other hand, if the temperature is less, the control systemperforms the variable ramp-down control at 208. Once the temperaturesetpoint is reached, the control system returns to routine 200 tomaintain the temperature at the temperature setpoint using a temperaturecontrol model (e.g., a PID control), at 210 and determines if there is anew temperature set-point, at 212. If there is a new temperaturesetpoint, the control system returns to 202. In one form, thetemperature setpoint can include a nominal setpoint when then heater isturned off.

Referring to FIG. 3, an example variable ramp-up control 300 isprovided. At 302, the control system sets the variable ramp rate to adesired ramp rate that is defined based on the temperature setpointand/or system current limit and provides power to the heater to achievethe desired ramp rate. At 304, the control system monitors the electriccurrent flowing through the resistive heating elements of the zones andthe temperature of each zone. At 306, the control system determines ifthe measured current for each zone is less than the electric currentlimit band. If so, the control system proceeds to 310. If not, thecontrol system, at 308, determines the reduction factor and reduces thevariable ramp rate for each zone based on the reduction factor. Inparticular, using the methodology described above, the control systemdetermines the reduction factor that is correlated to how close themeasured current is to the system current limit and reduces the variableramp rate by the reduction amount to obtain a permitted ramp rate, asthe variable ramp rate for each zone. At 310, the control systemdetermines if the temperature of adjacent zones are within a deviationthreshold to maintain a coherent temperature profile of the heater. Ifthe temperatures are within the deviation threshold, the control systemproceeds to 314. If at least one zone is deviating, the control system,at 312, reduces the variable ramp rate of the zone having the highertemperature, as provided above. Alternatively, the control system may beconfigured to boost power to the other zones while monitoring theelectric current of the zones. At 314, the control system determines ifthe zones are at the temperature setpoint. If not, the control systemreturns to 304. If the zones are at the temperature setpoint, thecontrol system returns to routine 200 of FIG. 2.

Referring to FIG. 4, an example variable ramp-down control 400 isprovided. At 402, the control system sets the variable ramp rate to acooling ramp rate (e.g., a second variable ramp rate) and controls thezones based on the cooling ramp rate. At 404, the control systemmonitors the temperature of each zone and at 406, the control systemdetermines if a temperature difference between the zones are within adeviation threshold to provide coherent temperature profile as theheater cools to the temperature setpoint. For example, the controlsystem determines if the temperature difference between adjacent zonesare greater than the deviation threshold. If the temperature differencesare within the deviation threshold, the control system proceeds to 410.If at least one zone is deviating, the control system, at 408, sets thetemperature ramp setpoint of the zone having lower temperature (i.e.,cool zone) to measured temperature of the zone having higher temperature(hot zone) and thus, increases power to the cool zone to achieve the newtemperature ramping setpoint. At 410, the control system determines ifthe zones are at the temperature setpoint. If not, the control systemreturns to 404. If the heater is at the temperature setpoint, thecontrol system returns to routine 200 of FIG. 2.

It should be readily understood that the routines 200, 300, and 400 canbe configured in various suitable ways and should not be limited tosteps described herein. For example, if the heater is a single zoneheater, the control system may skip steps related to providing coherenttemperature profile in routine 300 and may omit the variable ramp-downroutine. In another example, the VRRT control also includes a glidespeed control and/or an approach control to provide smooth transition tothe desired ramp rate and to the temperature setpoint, respectively. Inyet another example, for the variable ramp-down control, in lieu ofsetting a cooling ramping rate, the control turns off power to heaterand monitors the temperature of the zones to mitigate possible deviatingtemperature.

FIGS. 5A to 9 illustrate properties of the VRRT control of the presentdisclosure. Specifically, FIG. 5A illustrates a ramp-up operation inwhich the ramp-rate is constant (e.g., 20° C./min) and FIG. 5Billustrate a ramp-up operation using the VRRT control of the presentdisclosure. In both, the electric current is maintained under 30 A, butit takes longer for the constant ramp-up rate of FIG. 5A to reach 600°C. than the VRRT control of FIG. 5B. For the VRRT control, the ramp ratestarts at 28° C./min and is reduced as the electric current approaches30 A. That is, once the measured current is within an electric currentlimit band (e.g., 25-30 A), the ramp rate decreases to control theelectric current applied to the heater while allowing the heater toreach temperature setpoint.

FIG. 6 is a graph that illustrates ramp-up control in which the ramprate is controlled from a glide speed to the desired ramp rate, and thento an approach control rate when the measured temperature approaches thetemperature setpoint.

FIGS. 7A and 7B are graphs illustrating cooling of a two-zone heaterwithout variable ramp-down control and with variable ramp-down control,respectively. As illustrated in FIG. 7A, the zone temperatures start todeviate from each other, which can cause thermal stress, whereas, inFIG. 7B, the heater has a coherent temperature profile by addressing thedeviating temperatures.

FIG. 8 Illustrates the variable ramp-down control of the VRRT control inwhich the power is provided to the heater at a level slightly above theminimal amount (e.g., 5% power provided) to inhibit zone floatingcondition. By providing a small amount of power to the heater, thetemperature of the heater is continuously monitored and still decreasesto the temperature setpoint.

FIG. 9 illustrates the variable ramp-down control in which the runawayconditions are reduced or inhibited by controlling ramp rate and/orpower, as described above. In the figure, the filament temperature,which is representative of the heater temperature, and the temperatureramping setpoint (i.e., ramping setpoint (SP) in FIG. 9) aresubstantially the same during the ramp-down. In the figure, the optimalramping process variation (PV) reduction is reduction amount due todifferent zones deviating too much; the optimal ramp bottom reduction isamount of reduction due to power getting too low (float); the optimalramp net ramp setpoint (SP) gain is weighted summation of the threecorrective actions (e.g., the net gain is between 0.0 to 1.0 multiplieron the desired ramping SP to reduce the ramp rate, where 1.0 noreduction and 0.5 is 50% reduction); and the optimal ramp setpoint (SP)reduction, which may spike initially because the zone(s) may deviatefrom the ramp SP.

As used herein, the term deviation threshold generally captures thevarious possible threshold defined for comparing the difference betweena measured value (e.g., zone temperature, heater temperature) to anothervalue (e.g., a temperature setpoint, a temperature of another zone,etc.). In one form, the deviation threshold employed for monitoringzone-to-zone drift/deviation in the variable ramp-up control and thevariable ramp-down control maybe the same or different threshold value.In one form, for the variable ramp-down control, the deviation thresholdfor the zone-to-zone drift and ramp setpoint deviation may be the sameor may be different. In addition, the deviation threshold may beprovided as a singular absolute value (e.g., 5° C.) or is provided as arange (e.g., ±5° C.). The actual value of the deviation threshold isbased on the specific application and thus, is not limited to anyspecific numerical value provided herein.

Unless otherwise expressly indicated herein, all numerical valuesindicating mechanical/thermal properties, compositional percentages,dimensions and/or tolerances, or other characteristics are to beunderstood as modified by the word “about” or “approximately” indescribing the scope of the present disclosure. This modification isdesired for various reasons including industrial practice, material,manufacturing, and assembly tolerances, and testing capability.

As used herein, the phrase at least one of A, B, and C should beconstrued to mean a logical (A OR B OR C), using a non-exclusive logicalOR, and should not be construed to mean “at least one of A, at least oneof B, and at least one of C.”

In this application, the term “controller” may be replaced with the term“circuit”. The term “controller” may refer to, be part of, or include:an Application Specific Integrated Circuit (ASIC); a digital, analog, ormixed analog/digital discrete circuit; a digital, analog, or mixedanalog/digital integrated circuit; a combinational logic circuit; afield programmable gate array (FPGA); a processor circuit (shared,dedicated, or group) that executes code; a memory circuit (shared,dedicated, or group) that stores code executed by the processor circuit;other suitable hardware components that provide the describedfunctionality; or a combination of some or all of the above, such as ina system-on-chip.

The term code may include software, firmware, and/or microcode, and mayrefer to programs, routines, functions, classes, data structures, and/orobjects. The term memory circuit is a subset of the termcomputer-readable medium. The term computer-readable medium, as usedherein, does not encompass transitory electrical or electromagneticsignals propagating through a medium (such as on a carrier wave); theterm computer-readable medium may therefore be considered tangible andnon-transitory.

The description of the disclosure is merely exemplary in nature and,thus, variations that do not depart from the substance of the disclosureare intended to be within the scope of the disclosure. Such variationsare not to be regarded as a departure from the spirit and scope of thedisclosure.

What is claimed is:
 1. A method of controlling temperature of a heaterincluding a resistive heating element, the method comprising: applyingpower to the resistive heating element of the heater at a variable ramprate to increase temperature of the heater to a desired temperaturesetpoint, wherein the variable ramp rate is set to a desired ramp rate;monitoring an electric current flowing through the resistive heatingelement of the heater; and reducing the variable ramp rate from thedesired ramp rate to a permitted ramp rate in response to the electriccurrent being greater than a lower limit of an electric current limitband, wherein an upper limit of the electric current limit band isprovided as a system current limit.
 2. The method of claim 1, whereinreducing the variable ramp rate further comprises: determining areduction amount of the desired ramp rate based on a variable reductionfactor, wherein the variable reduction factor increases as the electriccurrent of the resistive heating element approaches the system currentlimit; and decreasing the variable ramp rate by the reduction amount tothe permitted ramp rate.
 3. The method of claim 2, wherein the variablereduction factor provides a scaled reduction of the variable ramp ratebased on proximity of the electric current to the system current limit.4. The method of claim 2, wherein the reduction amount is determinedbased on the followingRedAmt=(DesiredRate*% Reduction*RedFactor)% Reduction=1.0−((ZoneCurLim−MeasuredCurrent)/CurrentBand)  Equation 2in which: “RedAmt” is the reduction amount, “DesiredRate is the desiredramp rate, “RedFactor” is amount the variable ramp rate is reduced whenthe electric current is equal to the system current limit, “ZoneCurLim”is maximum electric current limit for the resistive heating element,“MeasuredCurrent” is the electric current that is measured, and“CurrentBand” is the electric current limit band.
 5. The method of claim1, wherein the heater includes a plurality of resistive heating elementsthat define a plurality of zones, wherein each of the plurality of zoneshas a defined variable ramp rate.
 6. The method of claim 5, wherein: theelectric current at each of the plurality of zones is monitored, and thevariable ramp rate is reduced from the desired ramp rate to thepermitted ramp rate in response to at least one zone of the plurality ofzones having an electric current that is greater than the lower limit ofthe electric current limit band.
 7. The method of claim 6 furthercomprising: determining a reduction amount for the at least one zonehaving the electric current greater than the lower limit of the electriccurrent limit band based on a variable reduction factor, wherein thevariable reduction factor increases as the electric current approachesthe system current limit; and reducing the variable ramp rate for eachof the plurality of zones based on the reduction amount to obtain thepermitted ramp rate for each of the plurality of zones.
 8. The method ofclaim 5 further comprising: monitoring a zone temperature for each ofthe plurality of zones; determining whether a difference between a firstzone temperature of a first zone from among the plurality of zones and asecond zone temperature of a second zone from among the plurality ofzones is greater than a deviation threshold; and adjusting the variableramp rate for the first zone, the second zone, or in combination thereofin response to the difference being greater than the deviationthreshold, wherein a zone from among the first zone and the second zonehaving a higher zone temperature is provided as a hot zone and the otheramong the first zone and the second zone is a cool zone.
 9. The methodof claim 8, wherein to adjust the variable ramp rate, the method furthercomprises: reducing the variable ramp rate for the hot zone; increasingthe variable ramp rate of the cool zone, or a combination thereof. 10.The method of claim 8, wherein to adjust the variable ramp rate, themethod further includes: reducing the variable ramp rate of the hot zoneto zero to hold the zone temperature of the hot zone until thedifference is no longer greater than the deviation threshold; andincreasing the variable ramp rate of the hot zone in response to thedifference being less than the deviation threshold.
 11. The method ofclaim 1 further comprising: setting the variable ramp rate to a glidecontrol rate, wherein the glide control rate is less than that of thedesired ramp rate; and increasing the variable ramp rate to the desiredramp rate in response to a glide condition being satisfied, wherein theglide condition include a predetermined time passing, the temperature ofthe heater equaling a glide temperature setpoint that is less than thedesired temperature setpoint, or a combination thereof.
 12. The methodof claim 1 further comprising: determining whether the temperature ofthe heater is at a temperature approach threshold, wherein thetemperature approach threshold is less than the desired temperaturesetpoint; and decreasing the variable ramp rate to an approach ramp ratein response to the temperature of the heater being at the temperatureapproach threshold, wherein the approach ramp rate is less than thedesired ramp rate.
 13. A control system for controlling power to aheater including a resistive heating element, the control systemcomprising: a processor; and a nontransitory computer-readable mediumincluding instructions that are executable by the processor, wherein theinstructions include: determining amount of power to be provided to theresistive heating element of the heater based on a variable ramp rate toincrease temperature of the heater to a desired temperature setpoint,wherein the variable ramp rate is set to a desired ramp rate; monitoringan electric current flowing through the resistive heating element of theheater; and reducing the variable ramp rate from the desired ramp rateto a permitted ramp rate in response to the electric current beinggreater than a lower limit of an electric current limit band, wherein anupper limit of the electric current limit band is provided as a systemcurrent limit.
 14. The control system of claim 13, wherein theinstructions further include: determining a reduction amount of thedesired ramp rate based on a variable reduction factor, wherein thevariable reduction factor increases as the electric current of theresistive heating element approaches the system current limit; anddecreasing the variable ramp rate by the reduction amount to obtain thepermitted ramp rate.
 15. The control system of claim 14, wherein thevariable reduction factor provides a scaled reduction of the variableramp rate based on proximity of the electric current to the systemcurrent limit.
 16. The control system of claim 13, wherein the heaterincludes a plurality of resistive heating elements that define aplurality of zones, wherein each of the plurality of zones has a definedvariable ramp rate.
 17. The control system of claim 16, wherein: theelectric current at each of the plurality of zones is monitored, and thevariable ramp rate is reduced from the desired ramp rate to thepermitted ramp rate in response to at least one zone of the plurality ofzones having an electric current that is greater than the lower limit ofthe electric current limit band.
 18. The control system of claim 17,wherein the instructions further includes: determining a reductionamount for the at least one zone having the electric current greaterthan the lower limit of the electric current limit band based on avariable reduction factor, wherein the variable reduction factorincreases as the electric current approaches the system current limit;and reducing the variable ramp rate for each of the plurality of zonesbased on the reduction amount to obtain the permitted ramp rate for eachof the plurality of zones.
 19. The control system of claim 17, whereinthe instructions further include: monitoring a zone temperature for eachof the plurality of zones; determining whether a difference between afirst zone temperature of a first zone from among the plurality of zonesand a second zone temperature of a second zone from among the pluralityof zones is greater than a deviation threshold; and adjusting thevariable ramp rate for the first zone, the second zone, or incombination thereof in response to the difference being greater than thedeviation threshold, wherein a zone from among the first zone and thesecond zone having a higher zone temperature is provided as a hot zoneand the other among the first zone and the second zone is a cool zone.20. The control system of claim 19, wherein to adjust the variable ramprate, the instructions further include: reducing the variable ramp ratefor the hot zone; increasing the variable ramp rate of the cool zone; ora combination thereof.
 21. The control system of claim 19, wherein theinstructions further include: reducing the variable ramp rate of the hotzone to zero to hold the zone temperature of the hot zone until thedifference is no longer greater than the deviation threshold; andincreasing the variable ramp rate in response to the difference beingless than the deviation threshold.
 22. The control system of claim 13,wherein the instructions further include: setting the variable ramp rateto a glide control rate, wherein the glide control rate is less thanthat of the desired ramp rate; and increasing the variable ramp rate tothe desired ramp rate in response to a glide condition being satisfied,wherein the glide condition include a predetermined time passing, thetemperature of the heater equaling a glide temperature setpoint that isless than the desired temperature setpoint, or a combination thereof.23. The control system of claim 13, wherein the instructions furtherinclude: determining whether the temperature of the heater is at atemperature approach threshold, wherein the temperature approachthreshold is less than the desired temperature setpoint; and decreasingthe variable ramp rate to an approach ramp rate in response to thetemperature of the heater being at the temperature approach threshold,wherein the approach ramp rate is less than the desired ramp rate.